The patent document No. JP-B-55-17958 has already described a watch case wherein a caseband-back of plastic material bears a disc-shaped back integrally formed with a portion referred to as the caseband. This caseband-back is overmoulded on the interior of a metallic bezel, the latter including attachment teeth rendering the whole impossible to dismantle once the injection of the plastic has taken place. In order to assure impermeability between the caseband-back and the bezel, there is arranged, prior to the injection, a packing mounted on a shoulder of the bezel. This packing is then compressed during the injection of the back thus assuring the desired impermeability.
This method of proceeding is not satisfactory for at least three reasons. Initially, the packing is likely to be damaged by the high temperature of the liquid material which is used to compress it. One might of course choose for such packing a material particularly resistant to the temperature involved, but this matter would not necessarily be the best as far as concerns the function to be assured, which is to say the impermeability of the watch case. As a further objection the proposed packing, surrounding as it does only two internal shoulders of the bezel, is not sufficiently localized or guided which may bring about its displacement or even its rupture at the moment of injection of the plastic material. Finally no matter how precise the mould, it leaves traces on the portions which must not be covered-here the visible portion of the bezel-this necessitating a subsequent cleaning or deburring and such operations may be considered as relatively costly.
Although concerning the impermeable fastening of a watch crystal of organic material to a watch case, one could employ in order to assemble a bezel onto a caseband, the procedure proposed by the French patent application Ser. No. FRA-2 491 644. This application proposes to provide the bezel with a groove turned towards the interior thereof. The crystal is supported on a shoulder of this bezel and there is applied thereto a heated punch having an annular form over the periphery of the crystal in order to plastically deform the latter and thus to cause penetration of a portion of the plastic material to the interior of the groove provided in the bezel. Conjointly therewith, there is compressed a packing between the bezel and the crystal. Such a procedure certainly enables the obtaining of an impermeable fastening of the crystal whilst liberating such of all mechanical fastening elements or any use of glue but it gives unsatisfactory results from the aesthetic viewpoint. Effectively, the deformation due to the application of the annular punch is produced over the front surface of the crystal. Moreover, such fastening techniques of the crystal bring into action non-negligible mechanical forces in order to obtain the deformation of the material. Such forces introduce internal tensions into the final product which alter the ageing qualities of the watch case provided with its crystal.
The purpose of the invention is to provide a watch case which is impermeable, reliable and of low cost. To this end there is provided a watch case including a caseband at least a peripheral portion of which is formed of plastic material, a back, a metallic bezel covering the caseband and extending over at least a part of the outer edge face of the caseband and impermeable assembly means for the caseband and the bezel, said assembly means comprising a first annular seat extending radially in the bezel, an annular protuberance integrally formed with the caseband portion of plastic material and extending radially into the interior of the first seat, said protuberance and said first seat being arranged in a manner such that the caseband and the bezel are inseparable, a second annular seat conjointly defined by said caseband and said bezel and a packing occupying said second seat.
The invention will now be explained in the description which is to follow given by way of example and with the help of the drawings which illustrate it.